1. Field of the Invention
The present invention relates to an electronic apparatus having a constitution that a capacitor is mounted on a mounting board, and more particularly to an improvement in inductance reduction of the electronic apparatus.
2. Description of the Related Art
Recently there has been a trend for communications equipment such as a cellular phone and information processing equipment such as a personal computer to be adapted to increasingly faster signals to deal with a huge amount of information. In keeping with this trend a CPU (Central Processing Unit) used in such equipment has come to operate at an increasingly higher clock frequency, wherefore harmonic noise is likely to occur. Furthermore, due to the presence of for example external noise resulting from peripheral apparatuses, circuits, and so forth of the aforementioned information processing equipment, the voltage supplied to the CPU is prone to contain a high proportion of noise.
Furthermore, there exists impedance in a power supply line for feeding a voltage to the CPU, etc. and ground. Therefore, in a case where the voltage supplied to the CPU, etc. contains noise, since voltage fluctuation occurs in the power supply line, it follows that a stable voltage is no longer supplied to the CPU, etc. This gives rise to problems including lack of stability in the workings of circuitry mounting the CPU, etc., occurrence of interference between different circuits through a circuit for feeding a voltage to the CPU, etc., and occurrence of oscillation.
In view of the foregoing, it is customary to connect a decoupling capacitor between the power supply line and ground. Moreover, the use of a capacitor having excellent impedance frequency characteristics is effective in obtaining enhanced decoupling effect. In this regard, a multilayer ceramic capacitor is smaller than an electrolytic capacitor in terms of ESR (Equivalent Series Resistance) and ESL (Equivalent Series Inductance (L)) as well. The multilayer ceramic capacitor is thus suitable for use as a decoupling capacitor. This is because, when ESL is small, the impedance existing in the power supply line and ground can be lowered, in consequence whereof there results no voltage fluctuation. Moreover, in a capacitor with small ESL, excellent noise absorption effect can be obtained over a wide frequency band. In order to enhance the decoupling effect of the capacitor, further ESL reduction will be necessary.
In that connection, in an electronic apparatus including a capacitor and a mounting board disclosed in Japanese Unexamined Patent Publication JP-A 2008-192808, the capacitor is composed of a multilayer body constituted of a plurality of insulating layers stacked on top of each other, an internal electrode formed within the multilayer body, and a terminal electrode formed on the outer surface of the multilayer body so as to be electrically connected to the internal electrode. The mounting board is composed of a board main body having a mounting surface, a connection pad formed on the mounting surface, and a through conductor formed within the board main body so as to be electrically connected to the connection pad. The internal electrode of the capacitor and the connection pad of the mounting board are disposed in face-to-face relation with each other. With the terminal electrode and the connection pad kept in an electrically-connected state, the capacitor is mounted on the mounting board. The through conductor is so disposed that, when projected on a virtual plane extending in a direction perpendicular to the mounting surface, or a direction in which the internal electrode of the capacitor is led out, an electric current flowing through the internal electrode is opposite in direction to an electric current flowing from the connection pad to the through conductor or an electric current flowing from the through conductor to the connection pad.
In the electronic apparatus thereby constructed, a magnetic field produced around the electric current flowing through the internal electrode and a magnetic field produced around the electric current flowing through the connection pad cancel each other out effectively, with consequent ESL reduction.
Incidentally, in the electronic apparatus as disclosed in JP-A 2008-192808, in order for the electric current flowing through the internal electrode to be opposite in direction to the electric current flowing through the connection pad in a planar view, the through conductor is so formed as to lie as close as possible to a side (hereafter referred to as “inner side”) of the connection pad in a planar view that is opposed to a side situated toward the end face of the terminal electrode mounted on the connection pad connected with the through conductor.
However, in the electronic apparatus as disclosed in JP-A 2008-192808, in the case of forming the through conductor in the vicinity of the inner side of the connection pad in a planar view, there is provided an extra current path extending from the vicinity of the inner side of the connection pad to a point immediately below that part of an end of the internal electrode which is exposed at the end face of the multilayer body in a planar view. Accordingly, even if magnetic field cancellation takes place, the influence of an increase in the length of the current path is unavoidable. This gives rise to a problem of an undesirable increase in ESL of the electronic apparatus.